This invention relates to a system for producing broadly tunable mid-infrared radiation.
The mid-infrared region of the optical spectrum is often referred to as the xe2x80x9cmolecular fingerprintxe2x80x9d region, because gases and vapors exhibit distinctive absorption features in this region. Examples of applications of mid-infrared absorption include pollution monitoring, atmospheric chemistry, and detection of presence of chemical and biological warfare substances. These applications and others can benefit from availability of compact and efficient mid-IR laser sources that allow detection and identification of trace gases and vapors.
Optical parametric oscillators (OPOs) are often the laser sources of choice where high peak power or average power, high conversion efficiency, and broad continuous tunability are required. Unfortunately, in the longwave part of the mid-IR region (xcex greater than 2 xcexcm), relatively few such lasers are available.
The long wavelength transmission cutoff of traditional nonlinear materials, such as lithium niobate (LiNbO3), potassium titanyl phosphate (KTP), lithium borate (LiBO) and beta-barium borate (BBO), has limited the tuning range of existing, commercially available OPOs to xcex less than 4 xcexcm.
On the other hand, chalcopyrites, such as ZnGeP2 (ZGP) and CdGeAs2 (CGA) exhibit very large optical nonlinearities, deep IR transparency (e.g., for xcex greater than 12 xcexcm) and high thermal conductivity and are the materials of choice for mid-IR OPOs for wavelengths beyond 4 xcexcm. These materials cannot be pumped by commercially available neodymium lasers (at xcex≈1.064 xcexcm), because of a lack of transparency at near-infrared laser wavelengths.
What is needed is a laser system that can provide broadly tunable infrared radiation in a wavelength range between about 2 xcexcm and about 17 xcexcm. Preferably, this radiation should be produced reasonably efficiently and should be capable of extension to higher wavelengths as well.
These needs are met by the invention, which provides a cascaded OPO system in which, for example, a Nd laser (continuous or pulsed) pumps a first stage OPO, such as PPLN, and the output radiation from the pumped first stage pumps a second stage OPO, such as a ZGP crystal cut 90xc2x0 to the optical axis of the second stage material, in a non-critical phase match configuration. The system has a relatively low pumping threshold and a relatively high efficiency